Introduce concept of temporary_retrieve_file_list to Parser class

[sphuber]

A common use case for Calculcation and Parser implementations might be that the calculation produces large files that one does not want to store permanently in the RetrievedFolder but it still contains valuable information that the user wants to parse and then store in output nodes. Currently, one has to choose to either not retrieve the files and hence not store the desired information or parse the information after the fact directly from the RemoteFolder by using something like an InlineCalculation.

An example would be to parse occupation numbers for a PwCalculation. Ideally one would parse these from the xml files, but they are large and otherwise pretty useless, so we do not want to store these by default. The current alternative is to parse them from the RemoteFolder with the get_bands_and_occupations_inline function.

If we were to introduce another file list in the AbstractCalculation class temporary_retrieve_list in which an implementation can specify certain remote files that upon retrieval will be stored locally temporarily, for the duration of the parsing call. After the parsing is done, be it successful or not, these temporary files will be deleted. During the parsing they will then be available for parsing just like normal retrieved files.